[TowerTalk] grounding radials: solid or stranded?

[Jim Lux]

----- Original Message -----
From: "Tom Rauch" <w8ji at contesting.com>
To: "Phil Camera" <kb9cry at comcast.net>; "William Q Meeker"
<wqmeeker at iastate.edu>; <towertalk at contesting.com>
Sent: Monday, March 14, 2005 6:01 AM
Subject: Re: [TowerTalk] grounding radials: solid or stranded?


> > > It has been suggested that stranded copper wire would
> have little less
> > > inductance and thus be more effective.
> > > Is there a clear choice or is this the trade-off that
> one needs to consider?
>
> Skin effect causes the outer strand in ANY stranded wire,
> even litz wire, to tend to carry nearly all the  high
> frequency current. Because it is a distance from center
> problem, the HF current mostly moves to the outer edges of
> each stand. The air gap between the stands caused by the
> radius of the stand means you have air occupying part of the
> available area instead of conductor. This makes the
> resistance of the wire increase.
>
> (Litz wire has higher resistance than solid wire, but it is
> useful in limited special applications where considerable
> external flux cross the conductor like multilayer
> transformers or inductors because it reduces eddy currents
> (shorted turn effect) in the winding's conductors.)
>
> Maybe I'm missing something, but I can't think of any
> application where stranding decreases resistance or reduces
> inductance in a given shape and size conductor. Can anyone
> think of a case? It isn't as good in RF coils, RF leads, or
> even at DC. It isn't as good in coax, it isn't as good in DC
> power leads absolute diameter and material being equal.


By spreading the current out into many filaments, Litz wire reduces the
losses due to skin effect compared to the SAME amount of the conductor in
single strand. A given current filament is farther from any other filament,
so the magnetic coupling is less. Essentially, you're increasing the surface
area, so the effective conductor area is increased (for a given amount of
copper).   This is fairly well covered in texts like Terman.

It's the same reason that if you take the same amount of copper and make a
tube, instead of a solid round bar, it will have less AC resistance, and why
wide flat strips have less resistance and inductance.

Woven braid, where all the strands are uninsulated, is an entirely different
story, although comparing the same number of strands in a solid bundle, vs a
woven layer around a dielectric core might be instructive.